In a Pressurized Water Reactor (PWR), the power level of a nuclear reactor is typically controlled by inserting and retracting control rods, which for purposes of this application include the shutdown rods of the reactor core. The control rods are moved by a Control Rod Drive Mechanism (CRDM), which typically include electromechanical jacks that raise or lower the control rods in increments. Known CRDMs include a lift coil, a moveable gripper coil, and a stationary gripper coil that are controlled by a Rod Control System (RCS) and a ferromagnetic drive rod that is coupled to the control rod and moves within the pressure housing. The drive rod includes a number of circumferential grooves at intervals (“steps”) that define the range of movement for the control rod. For example, a typical drive rod contains approximately ⅝ inch intervals and 231 grooves, although this number may vary.
The moveable gripper coil mechanically engages the grooves of the drive rod when energized and disengages from the drive rod when de-energized. Energizing the lift coil raises the moveable gripper coil (and the control rod if the moveable gripper coil is energized) by one step. Energizing the moveable gripper coil and de-energizing the lift coil moves the control rod down one step. Similarly, when energized, the stationary gripper coil engages the drive rod to maintain the position of the control rod and, when de-energized, disengages from the drive rod to allow the control rod to move. The RCS typically includes a logic cabinet and a power cabinet. The logic cabinet receives manual demand signals from an operator or automatic demand signals from Reactor Control and provides the command signals needed to operate the shutdown and control rods according to a predetermined schedule. The power cabinet provides the programmed DC current to the operating coils of the CRDM.
Known PWR designs are challenged in that they do not have adequate direct indication of the actual position of each control rod. Instead, step counters associated with the control rods are typically maintained by the RCS and rod position indication (RPI) systems to monitor the positions of the control rods within the reactor. The associated step counter is incremented or decremented when movement of a control rod is demanded and successful movement is verified. Because the step counter only reports the expected position of the control rod, certain conditions can result in the step counter failing and deviating from the actual position of the control rod. In certain situations where the actual position of the control rod is known, the step counter can be manually adjusted to reflect the actual position. However, if the actual position of the control rod is not known, a plant shutdown may be required so that the step counters to be initialized to zero while the control rods are at core bottom.
The RPI systems derive the axial positions of the control rods by direct measurement of drive rod positions. Currently both analog rod position indication (ARPI) systems and digital rod position indication (DRPI) systems are in use in PWRs. A conventional DRPI system includes two coil stacks for each control rod and the associated DRPI electronics for processing the signals from the coil stacks. Each coil stack is an independent channel of coils placed over the pressure housing. Each channel typically includes 21 coils, and the coils are interleaved and positioned at approximately 3.75 inch intervals (6 steps). The DRPI electronics for each coil stack of each control rod are located in a pair of redundant data cabinets (Data Cabinets A and B). Although intended to provide independent verification of the control rod position, conventional RPI systems are not accurate to fewer than 6 steps. For example, the overall accuracy of a DRPI system is considered to be about ±3.75 inches (6 steps) with both channels functioning and ±7.5 inches using a single channel (12 steps).
In contrast to conventional DRPI systems, conventional ARPI systems determine the rod position based on the amplitude of the DC output voltage of an electrical coil stack linear variable differential transformer. The overall accuracy of a properly calibrated ARPI system is considered to be about ±7.5 inches (12 steps). Neither conventional ARPI systems nor conventional DRPI systems are capable of determining the actual positions of the control rods. In the event of a step counter failure, plant shutdown for re-initialization of the step counter is required as the approximate positions of the control rods reported by conventional RPI are of little or no value.
For purposes of this application, the phrase “control rod” is used generically to refer to a unit for which separate axial position information is maintained, such as a group of control rods physically connected in a cluster assembly. The number of control rods can vary according to the plant design. For example, a typical four-loop PWR has 53 control rods. Each control rod requires its own sets of coils having one or more channels and the DRPI electronics associated with each channel. Thus, in a typical four-loop PWR, the entire DRPI system would include 53 coil stacks, each having two independent channels, and 106 DRPI electronics units. Further, in this application, the phrase “coil stack” is used generically to refer to the detector coils associated with each control rod and should be understood to include either or both channels of detector coils. Thus, a measurement across a coil stack contemplates the value across both channels combined and/or the value across a single channel.
Over time, aging and obsolescence issues have led to an increase in problems with conventional DRPI systems including analog card failures and coil cable connection problems that, in some cases, may result in unplanned reactor trips. These problems, along with plans for plant life extension, have prompted the industry to actively seek viable options to monitor the health and accuracy of the DRPI systems and/or to replace failing systems in order to ensure reliable plant operations for decades to come.
In addition to obsolescence concerns, the lack of diagnostic capabilities is a significant challenge. Since conventional RPI systems do not provide diagnostic information on their health other than the current rod position indication, diagnostics of the RPI system is limited to periods when the PWR is offline. The primary benefit of offline diagnostics is to catch obvious failures resulting from reassembly of the reactor. However, in between refueling outages, RPI failures can occur without warning, which leads to increased costs for the plant, especially if replacement parts cannot be obtained in a timely manner. Without active monitoring, plant engineers cannot identify problems developing in RPI systems and are unable to take preemptive actions, such as obtaining necessary replacement parts ahead of time and replacing failing components at the next scheduled outage. Instead, plants typically begin remedial actions after an actual failure occurs.
Beyond the technical challenges of controlling conventional DRPI systems, regulatory issues exist. Many existing PWRs are approaching the end of qualified life for several components of the conventional DRPI systems causing a demand for replacement options. There has been a significant push in recent years for plants to replace aging analog systems with digital systems made from commercially-available off-the-shelf parts. Using readily-available commercial parts provide plants more options for replacement in the future.